A major challenge for the development of mechanism-based therapeutic strategies to treat FXS is the diversity of FMRP target mRNAs, which encode proteins with a large variety of different functions in the neuron. Our recent work suggests that FMRP directly regulates intracellular signaling and protein synthesis through the PI3K/mT0R pathway, which is downstream of mGlu1/5 and other receptors, by limiting the translation and synaptic localization of pi 10p, a catalytic subunit of PI3K. The overall goal of this project is to provide experimental support for our hypothesis that dysregulated activity of the PI3K catalytic subunit p110 beta is a promising therapeutic target in FXS. These studies motivate aim 1 to investigate whether the selective genetic reduction or pharmacologic inhibition of p110 beta can rescue FX-associated phenotypes, as a novel therapeutic strategy acting at a key signaling molecule downstream of cell surface receptors. To reduce p110 beta activity, we, will (1) cross Fm1l knockout mice with mice heterozygous for p110 beta, and (2) use p110 beta selective antagonists. Since p110 beta selective inhibitors are in current clinical trials for cancers resulting from overactive PI3K, the proposed research may repurpose these available drugs for FXS. We will analyze the effect of p110 beta inhibition on FXS-associated molecular and cellular dysfunctions, and, in collaboration with Eric Klann (EAB Core), on impaired synaptic plasticity and behavior. To test the applicability of a p110 beta-based strategy in patients, we will analyze the effect of p110 beta selective inhibitors on PI3K activity and protein synthesis in induced pluripotent stem cells from FXS patients (aim 3). By integration with projects and rescue strategies from the Klann and Richter labs, and associated cores, we will employ genome wide ribosome profiling and bioinformatic analysis to test the hypothesis that inhibition/reduction of P110 beta (Bassell), S6K1 (Klann) and CPEB (Richter) reduces excess mRNA translation by correction of ribosome occupancy defects in Fmr1 KO mice (aim 2) and human iPSC-derived neurons from FXS patients (aim 3). This integrated and multidisciplinary approach will provide new insight into converging mechanisms of FMRP biology to reveal novel strategies to rescue FXS-associated phenotypes and lead to new therapeutic approaches.